Telephone-exchange system.



PATENTED MAR. 27, 1906.

uH. D.. sTRoUD. TELEPHONE EXCHANGE SYSTEM;

APPLIOATION ILBD MAR.25, 1901.

willigl Vix Z/arkg 6&7@ ZM PATENTED MAR. 27, 19.06. E

E o E. vD. sTEoUD. l

' TELEPHONE EXCHANGE SYSTEM.

APPLICATION FILED HA f PATBNTBD MAR. 27, 1906.

l H. 1)..STR0UD.' TBLEPHONBBXGHANGE SYSTEM;

I APPLICATION FILED MAR 25 1901 sHBETs-sHEET 3.

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OiNITEn STATES. PTENT OFFI'OE.

HAROLDv D. STROUD, OF CHICAGO, ILLINOISASSIGNOR, BY ivnesru;V

ASSIGNMENTS, To STRoMBEIie-OARLSON TELEPHONE MANUFAC- TUBING OOMPAN'nOF.'Rocnnsrna y'New YORK, A CORPORATION (OF NEW YORK.

'Specification of Letters Patent.

TELEPHONE-EXCHANGE SYSTEM.

' Application iiledMarnh 25, 19,01. Serial No. 52,788.

To- (tZ, `'wh/ont .it may concern: Be itknown thatl, HAROLD D. STROUD, acitizen ofthe United States, residing at Chicago, in the countyofCook'and State of Illinois, have invented a certain new and usefulImprovement Yin Telephone-Exchange Systems, of which the following isa'full, clear,

concise, and'exact description, reference being had to theaccompanyiI-idrawings formingua part of this specification".- y

invention /relates to change systems', and hasV for its object theprovision of improved meanswhe'reby subscrlbers may be broughtmtocommunication. The invention contemplates the provision of boards orsections of boards to' which subscrbers telephone-lines extend at someofwhich connectionsmay be partially completed, while at the remainingsections the connections may be completely established.

Generally speaking, the preferred vembodi-l ment oflone feature of myinvention consists in connecting each subscribers line with twodivisions of a board,gpro'viding line-signaling meansa't one of thesaiddivisions to enable the subscriber to initiate a call andconnecting'means extendin between the divisions of the board, in comination with means for conveying'intelligence between these divisions,so that an operator at one divisionof the board may lform acontinua-tion. of thel subscriberstelephone-line to the otherdivi'- sionof theboard, at whichlatter division the con.nection"between thesubscribers may be completed.

Another feature of my invention may be generally stated to consist in atelephone-ex- 'change system in which subscribers lines extend fromsubstations to an exchange, where the lines are divided up into a numberof groups, in combination withfmeans whereby a subscriber whose line isin one group may in sendin in a signal indicate to the operator aparticuigar group containing the line other subscriber with whomconnection is desired, switching apparatus bein interposed between thegroups and providey with means for conveying slglnals or other form of,intelligence between t e groups, whereby the o erators at these groupsmay cooperate in e ecting a completey connection. v

n the preferred embodiment of II1y'inven---l telephone exof an-.Patentedxrch 27, 190e. i

' tion the last feature thereof is preferably emi 'onedivisionfconstituting an answering divlsion, where thefconnection ispartially completed, and the remaining division constituting theeonnectin division, where Athe con nectIon between t e subscribers 1sfinished.

"Thus there are' for each setsr group of subscribers lines twoduplicated groups of connections at the' exchange. yIn other words, eachsubscribers line'is connected at the exchange to two springjacks,.thespring-jacks being connected in multiple, one spring-jack being locatedvupon ythe answering sect1on or divisionl of theboardand the other uponthe connecting-section of the bo'ard.- -Each sub- I scriber, ispreferably. provided with a device by which a selective device at theexchange vmay be operated to indicate to the answering operator thearticular roup of lines among '-which the ca led-for su scribers line isineluded.

lMy invention contemplates, further,` the provision of improvedsupervisory signaling appliancesto beemployed in the cord-circuitsextending between-the divisions of the board that may beincludeddirectly in the cord-circuit and yhave their operative conditioncontrolled by the subscribersapparatus and by thecord-connecting aparatus, the magnets of the supervisory signa s being propreferablyincluded in circuit when the parties are in conversation, so that theimpedance thereof is removed from the circuit by the OppOsitely-actingcoils of thel differential vided with differential coils, which are bothwinding and the signal is maintained quiescent at the same time, one ofthe coils being cut out'of the circuit when the signalis to'be conveyed,leaving the remaining coil to act alone, which thereupon mayv effect theoperation of a signal corresponding thereto.

I also provide means, in combination with the connecting cord-circuitsextendingbe;

tween the answering and connecting Sections.

vof the board, whereby the operator in selecting a plug 1of thecord-circuit extendin to the particular connecting-section where t econnection is' to be completed .may effect the opof the board in Fig. 3.

eration of a signal at the connecting-section to enable the operator atthe latter'section to select a corresponding plug, so that through theagency of these selected plugs at the answering and connectin sectionscomplete connection may be eii'ected between subscribers. Thecord-circuits extending between the answering and connecting sectionsare also preferably provided at the answering-section with anadditional` plug,-so that the operator at that section may complete aconnection if the called-for subscriber is in 'the same group with thecalling subscriber.

I will .explain my invention more fully by reference to theaccompanyingjdrawings, in whichn f -Figure 1 is a general diagrammaticview illustrating a number of Substations, one being shown connectedwith an answering and a .connecting station, 4while the remainder areshown connected with the connectingsections.V Fig. 2 is a generaldiagrammatic view- .illustrating the multiple arrangement ofeach pair ofsections, each pair includin an. answering and a connection section, anthe dispositionofa number of cord-circuits between the answering andconnecting sections. Fig. 3 is a detailed View of one form of select-iveline-signaling appliance to be operated by each calling subscriber, eachline having such an appliance. Fig. 4 is a plan view of thecharacter-wheel illustrated Fig. 5 is a view illustrating the mechanismtion for operating the selective device at the exchange 6 is a plan viewof part of the mechanism illustrated in Fig. 5, showing ardetentmechanism that may be employed. Fig; 7 shows a modified form ofcord-circuit 'and apparatus.

Like parts are indicated by similar characters of reference throughoutthe different figures. In Fig'. 1 I have illustrated six substationsbelonging to distinct groups. I Station A 100 may belong, say, to agroup of three hundred lines that may be connected iirst to .a board A,Figs. 1 and 2, being the connectingsection of the switchboard, and thenlto a switch in group A at the answering-section of the switchboard.Subscriber B 66 may likewise be one of a group-of three hundredalsohaving duplicated connections at the exchange, each line of this grouphaving oney connection at the board B group B. vvThe same with referenceto subscribers C 99, D 81, E 93, and F 76, each of which may be one of agroup of three hundred having similar connections at the exchange. Thuseach line is provided with two springjack connections at the exchangeoneat an answering-section and the other at the connecting-section. Becauseof peculiar test-signal apparatus, which I will hereinaflimbs of eachline.

located at each Subscribers staand at answering ter more fully setforth, the subscribers lines first extend to the connecting-sections ofthe board and thereafter to the answering-scctions; but I do not limitmyself to this arrangement.

Referring particularly to Fig. 1, the subscribers there illustrated havetheir lines (in this case each having two limbs) connected at theconnecting-board sections with the line-jacks a a a, which line-jacks inturn are connected to the jacks at corresponding answering-sections, thejacks at the connectingboard -A being connected with the jacks at theanswering-board A, the jacks at the connecting-board B' being connectedwith the' jacks at answering--board B, the jacks at connecting-board Cbeing connected with the jacks at answering-board C, and so onthroughout the series. There is located at each of theanswering-sections a selective signaling appliance b Afor eachsubscribers line which is normally connected with both Each selectivedevice comprises in one embodiment ofthe invention anoperating-electromagnet e, through which impulses -of currentmay beintermittently sent by suitable apparatus at the subscribersstation tobe hereinafter described. The armature of the electromagnet is providedwith a pawl f, that may upon each attraction of the armature rotate aratchetwheel g one step, the ratchet-wheel having iixedly securedthereto a character-wheel h, upon which letters or other suitablecharacter devices A, B, 0, &c., may be provided. A detent i preventsbackward rotation of the ratchet-wheel after each forward actuationthereof upon attraction of the armature connected with the pawl. Thelongerline-spring of the jack is preferably provided with a cam-block7n', that engages the inclined end lof the .armature when an IOOoperator engages a plug with the said linespring, which cam-block upondescending serves to release the pawlf from engagement with theratchet-wheel g and at the same time thrusts the detent '1l to one sideto permit the character-wheel to be restored to its normal position.

The device that I preferably employ at the subscribers stations foreffecting the stepby-step operation of the selective device at theexchange is illustrated in Figs. 1, 5, and 6. I have illustrated a'switch if, that is normally included in an open bridgebetween ,thesides `of the. telephone-line, this switch engaging as it is rotatedapluralityof contact-buttons (marked A, B 'C, D, E, F, which areelectricaily connected, but between which sufficient kspace exists sothat the varm, k in passing over the buttons will successively completeand open the bridge connection between the sides of ther telephone line,and thus correspondingly effect a step-by-step operation ofthe relayio uThe `answering operator thereupon willv pro-- ceed to have theconnection completed, as

answering .of the calling magnet e to present the proper character tothe operator to be seen through the opening Z. .Thus if subscriber A 100is a calling subscriber and desires to be brought intocoinmunication-With a subscriber of the F group the subscriber A'IOOwill rnove the switcharm ktothe button F, thus eecting six steps in therrevolution or" the character- Wheel to cause the character F upon thesaid wheel .to appear beforefthe opening will'be hereinafter set forth.In order that the switch crnay not be released until the 'tent fm by the'electromagnet n, that is ineluded in -the same bridge with theswitcharmV and the V(-:ontact-buttons.4 This electromagnet'n isdenergized when the operatorat the exchange plugs into thecalling'subscriberfs jack, the circuit including the said' magnet beingopened by the ren'ioval oit' the line-,springs of the jack from theirback contacts;` Ail suitable' signal' may be conveyed i to the-callingsubscriber when the answering operator plugs in, which may bein the formo .a signal-.bell o, that may be impinged upon by the switch-arm 7cwhen' the electromagnet n is deenergized.v The signal o, which -nf'raylbe either a visualy or audible si nalfis provided to'inform thesubscriber t at his connection is about to becom'pleted, whereafter thecalling subscriber may reniove his telehoneirom its switch-hook and givehis orer to the-connecting operator.

There islocated ateach answering-board section' a number. of`answering-plugs designed for insertion within the jacks o `Calle ingsubscribers, which. answeringplugs are C connected with connecting-plugsp located at the1connectin -fsections of theI switchf boards. Althoug vthere arein this instance six groups of answering-sections, .there arebut five connectingeplugs atthe c'onnectingsections of the switchboard,as-the answering i' operator may complete .connections betweensubscribers where the- -called subscriber. has

his line in the-same group with the calling subscribers line.' In orderthat .the answer: ing 'operatormay effect this connection betweensubscribers o fthefsarne group, addi-.

tional plugs g QL g2 g3g* arel connected in lnonl- 'tiple Withftheterminalplu s of 'the cord-circuit.' The. calling subscri verhaving oer-v ated `the Step-by-step selectiva device o his line to indicate the.group say group F--in lers line 1 o whichithe .called subscri ``s cated,

the answeringroperator selects a :cord-circuit' y sicher,

"indicator r at the connecting operators ,board. This signal 1preferably has its elecoperator has. pluggedinto the jack.

that a connection is to be establishe connecting operator thereuponwithdraws roe .which terminates in a connecting-plug' at theconnecting-section F, inserts 'the answeringplug p into the callingsubscribers jack, and by that act eiiects the operation of a suitabletrornagnet 4provided with 'diiierent'iai coiis, one 'included thetip-strand and the other in the sleeve-strand.. Viren both of the dii-'ferential coils are included in the circuit, the

coreof the electrornag net isnot energized l and no signal is effected.'The coil r is norrnally included in circuit with lthe tip-strand of thetrunking cord-circuit and with asignalconductor s, in` which latterconductor are included the plug-seat switch-contacts' s', that are`closed when theconnecting-plug isin its seat. This signal-conductorincludes contacts s2 at the answering-section,which arenormallyseparated when the terminal an.- sweringplug is in its seat. Abattery t, which may also .furnish current for the trans- `mitters atthe substations, isv included inseries with suitableimpedance devices,which, together with the battery, are included .in bridge between thestrands of the cord-circuit. lThe signal-cortidnctor-,s` is alsoconnected withthe said bridge conductor, so that when the'answering-plug p is removed 'fromits seat the 'circuit `of thesignaling-conductor 9.5

swill be coinpletedby the closure ofthe contacts s2, so that theindicator rwill be operated to indicate to the' connecting o eratlr theconnecting-plug from its seat and at the same time includesher-telephone set tin circuit with the cord circuit and the callingsubscribers apparatus and thereupon estabvlishes the connection that isdesired. The reg; l

connecting operator in removing the connectf ing-plugiro'rn its seatseparates th'e contacts s', s o thatthe same signal device which wasoperated to inform the connecting operator is thereafter placed incondition to serveias a 11o clearing-out indicatorito notify theconnect- `ing operator to disconnect when the subscribers are throughconversation. f The con'- necting operator thereupon inserts the saidconnecting-plug into the jack oi the called n r subscriber, which maybe,for example, F 7 6 o'f' the group Fr. There are located at theanswering-section of the switchboard two clearing-out indicators a u,one on each side of the bridge conductor, including .the battery t, 12owhich indicators are each provided with differentialjcoils through whichcurrent from the battery tiiows in opposite'directions, as it. alsoiiowsino posite directions through. the coils of the in icatorqo during thetime that i125 The" indisubscribers' are inl conversation.

' cators u 'u act'assupervisory indicators, so `that-they answeringoperator does notwithdraw the'answering-plug until both signals areoperated toindicate that both subscribers' 15o and circuit is closedthrough these operatingcoils from the ground u2, the battery t, thetip-strand of the trunking cord-circuit, the operating-coils of thesignals, thence to the tip sides of the telephone-lines to theswitchhooks and' the normally grounded contacts .engaging the same.- Ifa connection is to be completed between subscribers of the same group, Iprefer to employ a cam-lever o, that may be operated to separate theswitch-contacts s? to prevent the same from being closed upon theremoval of the answering-plug, sov

that the operator at the connecting-section of the board will notreceive a false signal.

In order to prevent the operators at any of the connecting-boards fromeffecting aconnection with a called/subscriber who may be connectedywith another subscriber at the same answering-section, I providetesting appliances wat the conneci'ig-boards, which may be includedserially 1n one side of the telephone line extending from eachsubscribers station to the answering-board. This test-indicator ispreferably in the form of an electromagnet provided with 'a smallshutter that may project slightly before the opening of thecorresponding spring-jack Iwhen the corresponding line is in use at theansweringsection, this indicator then receiving current from the batteryt. If the line is not busy, there being no current through the indicatorw, no busy sign will be manifested.

Referring now to Fig. 7, I will describe an improved form of cordconnecting apparatus that may be used in connection with the systernillustrated in Figs. 1 to 6, inclusive, or in connection with ordinaryswitchboards or single boards. Two telephone-lines from stations A 100and A 96, extending to the same board, are illustrated, and the plugs pand gare linserted within the jacks of these lines. The sleeve-strand ofthe cord-circuit includes the operating-coils u of the su ervisorysignals u u, while the tip-stran includes the counteracting coils a2.The battery tis included in a bridge between the strands of thecordcircuit and between impedance -coils also included in the samebridge. rThe sleevestrand of the cord -circuit includes also the helicesof magnets w w, whose armatures are provided with plug-seats w2'w2,which, through the agency of the seated plugs, remove the other endsof the armatures from'contact-anvils w3, that constitute the terminalsof a signaling-conductor w4, connected with the battery t. When thecircuit is open at a subscribers station by the telephone being-upon itsswitch-hook thereat, the armature of the corresponding electromagnetwillv be permitted to fall against its contact w3, whereupon circuitwill be closed through the corresponding operating-helix u to effect theactuation of the indicator u, which is permitted because thecounteracting coil u2 is included in open circuit on account of theposition of the switch-hook at the corres ending Subscribers station.Thus a 'contro of the ksupervisory'signals may be eected without lampsincluded serially in the conductor w4 where desired,v rlhe simplecord-circuit illustrated in Fig. 7 may be brought into trunk connectionwith a connecting-plug at a distant section of the switchboard by havingan answering-plug q inserted within a trunking-j ack y. y

I have not deemed it essential to describe in detail any particular formof telephonie apparatus at the substations. I have shown at station A100 a bell 'y' included between the sides of the telephone-line with acondenser The transmitter 2 may be included in series with thetelephone-line when the telephone-receiver z is removed from thetelephone'sWitch-hook 22, the arrangement being suchthat when thetelephone-receiver is upon its hook the sides of the telephoneline arenot conductively connected, the condenser being the preferred meansemployed for preventing a iow of current over the telephone-line whenthe substations are idle.

.It is obvious that changes may readily be made in the preferredembodiment of my invention herein shown and particularly described, andI do not, therefore, wish to be limited to the precise disclosure of theinvention herein set forth; but,

Having thus described my invention, I claim as new and desire to secureby Letters Patenty l. The combination with `telephone-'lines extendingfrom subscribers 'stations and passing serially through two boardsat theexchange, each line having a switch connection withieach board, oftrunkcircuit connecting means interposed between the boards,line-indicators at one of the boards, test-indicators associated withthe telephone-lines at the other board, a source of current for thetrunk -'circuit, and means whereby the said test -indicators areoperated by said source when the trunkcircuit is connected. with theline at the first board, connection of said circuit also mechanicallycausing restoration of the line-indicator.

2. The combination with telephone-lines extendingfrom subscribersstations and Vpassing serially through two boards at the exchange, eachline having a switch connection with each board, of connecting meansinteiposed between the boards, line-indicators 'at one of the boards,test-indicators associated with the telephone-lines at the other board,and means whereby the said test-indicators' nullifying themagnet-izingeilect of ,of an'indicator at one terminal .of the trunk ingconnection having a controlling-electro-y cludedin opposite sides 'ofthe said of an indicator at one terminal of the trunking connectionhaving a controlling-electro of the saidlcoils toeifectv an operation ofthe 4 operated by the terminal plugs, the -plug at one terminal of thecord-circuit serving to 'the said test-'indicators being located.'to-the Arear of .the plugopenings and presenting ar scribed.

' through one of the saideoilsto effecty an op-` 4theiirst aforesaidterminal of thel trunking vof the fcontrolling-electromagnet being inj-Braies' are operated when thelines at theliirst board are l connectedywith the `connecting means,

signalbefore the plug-openings when thecorresponding line is busy toguard against the insertionof plugs, connectio n of said connectingmeans with the linealso causingmechanical engagement `with the"l'inefindicator to cause restoration thereof substantially as de- 3. Ina trunking cord connectionth e combination with the strands of thetrunk-cord, ofan indicator at one terminal ofthe trunk# ing connectionhaving a controlling-electro magnet provided with dii'ferentially-woundcoils, means at the other terminal of the trunking connection forclosing circuit through one of the said coils toefect anoperation'of thesaid indicator, andmeans' at the first aforesaid terminal of thetrunking connection for. the said winding, substantially as described.'4. In a trunking cord connection, the combination with the strands ofthe trunk-cord,

magnet provided with d iiierentiallywound coils, fm'eansat the otherterminal ofgthe trunking connection wfor closing circuit eration of the`said indicator, andineans' at connectionfor nulliiying themagnetizingeffeet of the said winding, thedifferential coilscordecircuit, substantially as described.

5. In a trunking-cord'co'nn'ection, the combination with the strands ofthe'trunk-cord,

magnet provided `with differentially-wound coils, means at the otherterminal of the trunk' ingconnection for c lo'sing circuit throughonesaid indicator, and. means at'the 'iirst aforesaid terminal of thetrunking connection for opening the circuit including the said wind-Iing, substantially as described.

6. The combination with telephone-lines extending from substationsto-different boards of the exchange, of a'trunking cord-circuitinterposedbetween the said` boards and provided with terminal plugs, twosignals, one at each terminal of the cord-circuit, each signal beingprovided with a controlling-electromagnet having differentially-woundcoils 'included'in` opposite sides oi' the cord-circuit, asignaling-conductor, and plug-seat switches close circuitthrough'asingle operating-coil of the controlling-magnet at the otherterminal of the cord-circuit, while theterminal `circuit serves upon itsremoval toe ect an a signaling-circuit including't'he saidindicaf.tors", .faiidvswitches controlled by the arma# at the latter endfoi thetrunkin cordoperationv of the switch controlled thereby' to `open thecircuit includingthe said coi, substantially as'described; 7. `In atelephone-exchange system, the v ioinbin'ation with telephone-linesextending rom subscribers stations to diiferent boards t the exchange,of a trunking cord-circuity t `the exchange 'provided with threesignals, each havinga controlling-electromagnet havingdifferentially-wound coils, the coils of each magnet being included inopposite sides of the cord-circuit, two of the said 'signals being1ocated at one of the'terminals of the cord-circuit,while 'the remainingsignal is included at v the remaining terminal ofthe cord-circuit,.and la source'ofcurrent included between the magnets of the signals at oneterminal of th'ecordcircuit and in bridge between vthe sides of the.cord-circuit,substantially as described.

8. `In a telephone-exchan e system, the@v combination with telephoneinesextendin from subscribers stations to an exchange, o a cord-circuit torconnecting subscribers for provided with plug-seat switches adapted toreceive the terminal plugs of the cord-circuit,

conversation, indicators for the cord-circuit, `two electromagnetshavingtheir armatures' tures of the said electromagnets, the said` plugsin engaging. the seats provided upon the' being controlled by theapparatus at the substations, i whereby circuits through l the saidindicators may be closed `and 'opened whilethe plugs are in line-jacks,substantially as described.

`armatures servingto open the signaling-'circuit, the circuit vthroughthe electromagnets 10Q` -9. In an operators apparatus, the combinationwith an electromagnet of an armature,

of a plugse'at provided in the armature yfor i reeeivingaplug, and aswitch-contact operatively associated with the said armature, sub1vstantially as described.-

10. In fa telephone-exchange system, 'the combination withtelephone-lines extending from subscribers" stations to diiierent boardsat the exchange, of a trunking cord-circuit intervening between theboards, the said trunking cord-circuit' being provided. nearone terminalthereof with two onnecting f plugs, whereby a connection may be oomvpleted between subscribers whose lines ,ex-i

tend to the saine board, the other end` of the trunking` cord-circuitbeing provided withl Y suitable l connecting. means, whereby 'sub'-scribers linesextending to diHerent boards maybe connected forconversation, avsignal of the plugs at` the first aforesaid end ofthe ftrunking cord-circuit to. eHect the operation located at the latter endof the trunking cord-' circuit, a switching device controlled by onenected wit the o eration of the said switching device when inesextending to thesame board are to be united for conversation through theagency of the said pair of connecting-.plugs- `at one terminal of thecord-circuit, substantially as described.

11.. In a telephone-'exchange'system, the" spring-jack lthereat and withthe line, the insertion of a plug 1n said spring-j ack serving tointerrupt the line and to disconnect said signal therefrom,substantially as described.

12. In a telephone-exchange system, the combination with telephone-linesextending from substations to an exchange, each line passing through aconnecting-board and terminating at an answering-board, of springjacksand lplug-o enings at each board consaid iines, and indicating-si nalsat the connecting-board included serial y in the lines and normallyconnected with the springjacks, each signal serving when the subscribertelephonically connects himself with the corresponding line topreventthe insertion of a plug into the corresponding plugopeninsubstantially as described.

13.- n a telephone-exchange system, the combinationwith telephone-linesextending iro subscribers stations and passing seria ly throughswitchboards at the exchange,

of a-trunk-circuit between the switchboards for uniting subscribers forconversation, differentially wound electromagnet signaling mechanismincluded in said trunk-circuit,

from subscribers substations and passing 'serially through-switchboardsat the exchange,

of a trunk-circuit between said switchboards for uniting subscribers ofone board with subscribers of the other board,differentiallywound-electromagnet signals and a source of currentincluded in said trunk-circuit, and means independent of saidswitchboards and the telephone-lines for causing actuation of saidsignals to convey intelligence between the operators at said boards.

.15. In a telephone-exchan e system, the combination with telephoneineseach extending from a subscribers station and passing serially throughtwo switchboards at the exchange, of a trunk-circuit between saidswitchboard for uniting the subscribers of one board with thesubscribers at theotherboard, differentially-wound ele'ctromagnetsignals and asource ofcurrent included in said trunkcircuit, means uponconnection of said cordcircuit with the telephone-lines and uponactuation of substation apparatus for indicating to the board operatorsthe condition of the line and substation apparatus, and means in saidAtrunk-circuit entlrely independent of the connection of' saidtrunk-circuit with the lines for causing actuation of said signals toconvey intelligence between the operators at the boards.

16. In a telephone-exchange system, the combination with telephonelineseach extending from a subscribers station and terminating at aswitchboard at the exchange, of a trunk-circuit between thelswitchboards at the exchangev for uniting subscribers of one board withsubscribers of another board, differentially wound electromagnet signals'and a source of current includedin the trunkcircuit, and switchin meansassociated with the plugs of the trun -circuit for causing actuation ofsaid signals independent of the switchboards and telephone-lines wherebyintelligence may be conveyed between the operators at the differentboards. In witness whereof I hereunto subscribe my name this 14th day ofMarch, A. D. 1901.

HAROLD D. STROUD.

Witnesses:

GEORGE L. CRAGG, HARVEY LHANsoN.

